The process of regasification of cryogenic liquefied gases and liquefied natural gas (LNG) is a well-known commercially practical process. Indeed there are several different commercial methods for carrying out the process, each process using a different source of heat for regasification. These are generally ambient air vaporizers, seawater (open rack) vaporizers, and water bath vaporizers using submerged combustion or immersed fire tube heaters.
The disadvantage of the method using the combustion of fuel as the heat source is the cost of the fuel required, the complexity of the process and the environmental consequence of the combustion itself. The seawater heat source type has the disadvantage of limited availability, an adverse effect on marine life and the corrosive effect of seawater on the materials used in the process.
In the case of ambient air vaporizers, the air as the heat source is readily available, environmentally and economically favorable and non-corrosive to the materials used. The disadvantages of using ambient air as the heat source for LNG regasifiers and cryogenic vaporizers in general are that the heat exchangers are relatively large and are limited by the temperature of and the humidity within the atmospheric air at any particular location. These problems have been partially solved by the implementation of various configurations of ambient air vaporizer heat exchangers and how individual heat exchange elements are made.
In the case of ambient air vaporizers, ambient air is the regasification heat source, which the present invention is concerned, the cryogenic liquefied gas is passed through a vaporizer comprised of an array of externally finned vertical heat exchanger tubes to heat, vaporize and superheat the cryogenic liquefied gas.
The atmospheric vaporizer in U.S. Pat. No. 4,399,660 to Vogler et. al., 1983 Apr. 23, shows a multi-pass up-down configuration defined by a critical pass heat exchange element spacing ratio. While Vogler claims continuous operation for his device, the data presented covers only a six (6) day period (Col 6, lines 14-17) of operation. Subsequent use of this configuration has shown that beyond the 6 day period performance continuously declines due to continuous ice build-up. The atmospheric heat exchange element in U.S. Pat. No. 5,350,500 to White et. al., 1995 Feb. 21 attempts to mitigate ice build up as described in Vogler. Vogler fails to instruct on the potential benefit of additional external fins beyond eight (8) possibly due to his focus on long-term ice build-up (col. 7, line 1 and col. 7 lines 18-19). White discusses switching vaporizer heat exchanger banks (col. 2, lines 27-37) to achieve continuous operation, yet he fails to fully explore conditions whereby switching may offer improvement over non-switching atmospheric vaporizers by moderating the heat transfer process for which he instructs.
The single pass ambient vaporizer in U.S. Pat. No. 5,251,451 to Weider 1993 Oct. 12 offers the improvement of counter-current flow of the air to the flow of cryogenic fluid, a well-known heat exchanger design condition. As with parallel tube heat exchangers and more particularly with boiling fluid and cryogenic vaporizers, flow maldistribution within the multiplicity of parallel flow tubular passages is compounded by the two-phase flow region as described in U.S. Pat. No. 4,083,707 to Bivins 1978 Apr. 11. This condition is dealt with by Weider by inserting a solid rod within the fluted interior of the vaporizer heat exchange element described. Weider restricts the application to an internal fluted geometry wherein the ratio of the exterior surface area to the internal, fluted, surface area is within the range of 5:1 to 25:1 (col 4, lines 23-29), restricts the use of this art to lower pressure cryogenic fluids for reasons not described and does not instruct that in stainless steel lined externally finned elements the area ratio as he defines may be in the range of 50:1 to 125:1. In U.S. Pat. No. 5,473,905 to Billman 1995 Dec. 12, a modified rod insert of a type described by Weider, for the purpose of surge control (col. 2, line 38) and the limitation of these type inserts to lower pressure cryogenic fluids is described. For higher pressure and higher pressure drops, Billman points out “twisted tape turbulators” are not always beneficial (col. 2, lines 27-33) but he apparently fails to realize that the vortex or swirl flow created by such inserts provide improvement in heat transfer at lower pressure drop for boiling or vaporizing fluids at any pressure. Billman combines different lengths of solid and hollow tube inserts in combination which require an internal fluted tube geometry with restricted internal geometries for both cross sectional fluid flow area and internal to external perimeters (surface area ratios). Billman fails to instruct on alternate means of controlling flow maldistribution, which do not require the increase of pressure drop as do the solid rod inserts he teaches for this purpose. Billman further instructs that for his invention “no significant heat transfer” (col. 4, lines 18-20) and “minimal heat transfer” (col. 4, lines 45-49) takes place at specific locations, which, minimal heat transfer however reduces heat exchanger efficiency.
As natural convection ambient air vaporization systems have become larger to meet the commercial need of higher regasification flow rates prior art has shown little appreciation for the need to be concerned that the air is the heat source for the vaporization/regasification process and that the free flow of air to an exposed ice surface is critical. Vogler as cited above instructs a ground clearance of 2 to 4 feet (col. 2, lines 57-58) and is primarily concerned with ice buildup (col. 1, lines 37-40). In U.S. Pat. No. 4,566,284 to Werley 1986 Jun. 28, is discussed improved positioning of flow passages in a vertical all series cross flow atmospheric vaporizer. This art would not apply to the vertical, parallel pass, counter flow vaporizer of this invention and does not allow full benefit of the chimney effect created by tall vertically installed all parallel heat exchanger elements.
In U.S. Pat. No. 4,479,359 to Pelloix-Gervais 1984 Oct. 30 shows an atmospheric heater for cryogenic fluids of “higher heat exchange efficiency” (col. 1, lines 25-28). Particularly, the air flow passages of the individual heat exchange elements are discussed (col. 1 lines 57-60). The inner fin configuration with the preferred all aluminum heat exchange element in col. 1, lines 35-40 limits the configuration to lower pressure cryogenic fluids. The attempt by Pelloix-Gervais to gain an increase in overall heat gain by painting some external portions black is noteworthy, however, in large regasification systems, the percent of the total heat transfer surface which can be profitably exposed to solar radiation is to the order of 1% thus limiting any solar gain to be well below ¼%. For this reason, naturally oxidized aluminum is the outer surface material preferred, by present art. Pelloix-Gervais does not instruct upon the effect of ice thickness on heat exchanger efficiency or configuration.
It is well-known that in the particular case of heat exchangers wherein boiling and vaporization take place within the tubular passages that tube inserts offer advantage as described in the aforementioned art. Inserts of many configurations are illustrated in prior art. In U.S. Pat. No. 5,341,769 to Ueno et al 1994 Aug. 30 shows seven insert configurations to improve the regasification in seawater LNG vaporizers. The falling (water) film counter current heat exchanger panels described tend to have ice build-up, which Ueno attempts to mitigate with the insulated type insert described. Also described is pressure contact for controlled heat transfer (col. 4, lines 23-25). In U.S. Pat. No. 4,296,539 to Asami 1981 Oct. 27 a particular twisted spoke type insert for the improvement for water spray natural gas evaporators (col. 2 lines 12-21) is shown. Asami further instructs on the film boiling aspect of cryogenic vaporizers (col. 1, lines 37-56) and for the particular case described, the helix configured has a defined twist ratio between 5 and 15. Asami apparently fails to appreciate the relationship between his defined twist ratio and the centrifugal separating force required between the evaporated fluid and the fluid liquid droplets described, especially for the case of LNG, which is not a pure fluid, but rather a mixture of components which do not boil at the same temperature producing lower rates of heat transfer. His defined twist ratio results in reduced separation of fluid phases due to the relatively large internal diameter of the tube, which is in the case described between 10 to 20 cm or between about 4 to 6 inches. Asami also fails to instruct the effect of pressure on the internal heat transfer process. As pressure increases, the contrast in the density of the fluid evaporated portion and fluid liquid portion diminishes by the well-known laws of thermodynamics, requiring an increased centrifugal force to effectively separate the two fluid phases for the purpose of higher heat transfer during vaporization. It is well known to those skilled in the art that lower twist ratios, which as defined results in more twists per foot of length, improve boiling heat transfer over higher twist ratios. In a process described in U.S. Pat. No. 6,664,432 B2 to Ackerman 2003 Dec. 16 an internal insert combined with a reaction catalyst is described as being effective using an insert causing a pressure drop increase of not more than three times that of a bare tube (col. 3, lines 3-8). The inserts described by Ackerman apply to a particular retrofit process (col. 3, lines 59-61) thereby requiring restrictions which do not apply to processes operating at higher pressures, different temperature ranges, for different reasons, or where significant vapor superheats are required.
Although the use of seawater or atmospheric ambient air offers the advantage of not requiring added heat from a source such as fuel combustion, controlling the seawater or atmospheric air is of a particular concern. In U.S. Pat. No. 6,089,022 to Zednik, 2000 Jul. 18, seawater is pumped through a heat exchanger on board a ship to regasify LNG. As instructed by Zednik (col. 5, lines 30-40) the relationship between where the seawater intake is located and where the seawater is discharged back into the sea is defined for efficient operation. Although the positioning described by Zednik for seawater is instructive, the differences between seawater and atmospheric air in a natural convection heat exchanger requires a different solution for ambient air vaporizers.
In heat transfer processes where the heat flow passes through two or more materials in contact with one another, the contact surfaces are a source of inefficiency. Generally, as described above, those skilled in the art attempt to provide intimate contact by deformation, intimate pressure contact and the like. In U.S. Pat. No. 4,487,256 to Lutgens et al 1984 Dec. 11 there is described a cryogenic ambient air heat exchanger employing a pair of externally finned elements clamped onto a smooth inner fluid conduit tube to maintain intimate contact at the mating metal surfaces. Likewise, in U.S. Pat. No. 3,735,465 to Tibbets 1973 May 29 a related clamping system is described. U.S. Pat. No. 4,598,554 to Bastian 1986 Jul. 8 discloses a stainless steel finned element vaporizer system with the fins welded and bonded to the stainless steel horizontal tubes for the particular purpose of structural rigidity (col. 3, lines 41-45). What this prior art apparently fails to realize or does it instruct is that due to the surface imperfections within the metal mating surfaces when these surfaces are brought into intimate contact they contain air pockets or cavities which considerably restricts the free flow of heat between the metal surfaces. It is known that elimination of these cavities between the mating surfaces can reduce the contact resistance to heat flow between the surfaces by 20 times more or less depending upon the process used. Tibbets and Lutgens fail to instruct in this regard nor do they teach that contact pressures above 1000 PSI do not fully eliminate the resistance to heat flow caused by the mating surface area pockets.
Now it is realized by those skilled in the art of heat transfer that the heat from the air must pass from the air to the cryogenic fluid through a series of resistances to the free flow of the heat required. These include: free air flow, air to ice layer, through the ice layer to the metal surface, down the fin length, through the metal hub, through the contact resistance between hub and higher pressure tube surfaces, through the high pressure tube wall, through the fluid to tube wall boundary layer and finally into the temperature gradient within the vaporizing fluid. To improve the total heat transfer process, each of these elements of the heat transfer process needs to be evaluated and improved in order to offer improvement of the overall heat transfer process essential to the regasification process.
Accordingly, there is a need for a natural convection ambient air cryogenic vaporizer regasification process and method, which improves the free flow of atmospheric air, increases the total heat transfer performance and provides a more compact and economical vaporizer heat exchanger, which operates continuously.